docs: def nucmer_hits_to_ref_coords(cls, nucmer_hits, contig=None):
          '''Input is hits made by self._parse_nucmer_coords_file.
             Returns dictionary. Key = contig name. Value = list of coords in the
             reference sequence for that contig.
             if contig=contig_name, then just gets the ref coords from that contig,
             instead of using all the contigs'''
          coords = []
          if contig is None:
              coords = {key: [] for key in nucmer_hits.keys()}
          else:
              coords = {contig: []}
  
          for key in coords:
              coords[key] = [hit.ref_coords() for hit in nucmer_hits[key]]
              pyfastaq.intervals.merge_overlapping_in_list(coords[key])
  
          return coords